POST /api/artwork/uploads (multipart/form-data) accepting logo and
watermark images. Validates content type (png/jpeg/gif/webp) and size
(SystemEnvironment.MaximumUploadMb, default 10MB) mirroring the Blazor
upload path; stores via IImageCache.SaveArtworkToCache; returns
{ path, contentType } consumable by channel create/update.
Includes handler + controller tests, OpenAPI 422 contract-test entry,
and regenerated v1.json.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
